Other Comprehensive Income - Accumuldated Balances for each classification (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances		$ 2,012,321	$ 1,882,266	$ 2,137,513
Deferred income tax benefit (expense)		82,614	(85,512)	(98,524)
Balances		2,449,589	2,012,321	1,882,266
Interest expense		(30,943)	(33,705)	(38,588)
Unrealized holding gains / losses arising on fixed maturities, available-for-sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances	[1]	311,748	339,520	784,183
OCI before reclassifications	[1]	377,605	20,295	(643,880)
Deferred income tax benefit (expense)	[1]	(132,162)	(7,103)	225,358
AOCI before reclassification, net of tax	[1]	245,443	13,192	(418,522)
Amounts reclassified from AOCI	[1]	(29,710)	(63,022)	(40,217)
Deferred income tax benefit (expense)	[1]	10,399	22,058	14,076
Amounts reclassified from AOCI	[1]	(19,311)	(40,964)	(26,141)
Impact of tax reform	[1]	7,007
Balances	[1]	544,887	311,748	339,520
Unrealized holding gains / losses arising on cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances	[2]	67,076	45,284	33,141
OCI before reclassifications	[2]	(73,726)	44,776	31,061
Deferred income tax benefit (expense)	[2]	25,804	(15,672)	(10,871)
AOCI before reclassification, net of tax	[2]	(47,922)	29,104	20,190
Amounts reclassified from AOCI	[2]	(4,498)	(11,249)	(12,380)
Deferred income tax benefit (expense)	[2]	1,574	3,937	4,333
Amounts reclassified from AOCI	[2]	(2,924)	(7,312)	(8,047)
Impact of tax reform	[2]	87,299
Balances	[2]	103,529	67,076	45,284
Unrealized holding gains / losses arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Interest expense		2,954	2,657
Future policy benefits, DAC and VOBA adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances		(58,646)	(65,785)	(108,194)
OCI before reclassifications		(88,648)	10,983	65,245
Deferred income tax benefit (expense)		31,027	(3,844)	(22,836)
AOCI before reclassification, net of tax		(57,621)	7,139	42,409
Amounts reclassified from AOCI		0	0	0
Deferred income tax benefit (expense)		0	0	0
Amounts reclassified from AOCI		0	0	0
Impact of tax reform		0
Balances		(116,267)	(58,646)	(65,785)
Employee benefit plan adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances	[3]	(84,303)	(85,581)	(106,112)
OCI before reclassifications	[3]	7,283	(7,315)	19,730
Deferred income tax benefit (expense)	[3]	(2,549)	2,560	(6,905)
AOCI before reclassification, net of tax	[3]	4,734	(4,755)	12,825
Amounts reclassified from AOCI	[3]	6,982	9,281	11,856
Deferred income tax benefit (expense)	[3]	(2,444)	(3,248)	(4,150)
Amounts reclassified from AOCI	[3]	4,538	6,033	7,706
Impact of tax reform	[3]	(16,161)
Balances	[3]	(91,192)	(84,303)	(85,581)
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balances		235,875	233,438	603,018
OCI before reclassifications		222,514	68,739	(527,844)
Deferred income tax benefit (expense)		(77,880)	(24,059)	184,746
AOCI before reclassification, net of tax		144,634	44,680	(343,098)
Amounts reclassified from AOCI		(27,226)	(64,990)	(40,741)
Deferred income tax benefit (expense)		9,529	22,747	14,259
Amounts reclassified from AOCI		(17,697)	(42,243)	(26,482)
Impact of tax reform		78,145
Balances		$ 440,957	$ 235,875	$ 233,438

[1] Reclassifications affect Other realized investment gains(losses), net on the consolidated statements of income.

[2] Reclassifications affect net investment income on the consolidated statements of income, except for $2,954 and $2,657 (before tax) which affected interest expense for the years ended December 31, 2017 and 2016, respectively.

[3] The adjustments for defined benefit plans are included in the computation of net periodic (benefit) cost of employee benefit plans (see note 17 for additional details).